Selection of materials for some applications, such as packaging or electronics for example, requires studying permeation of some gases through these materials. Permeation refers to the mechanism by which a gas passes through a material in several steps, namely absorption of the gas in the material, diffusion of this gas through the material, and desorption of the gas on the other side of the material. The most commonly used permeation measurements generally concern to gases such as oxygen or water vapour through materials to be selected.
For example for materials intended to be used for agri-food packaging, it is critical to study permeation of common gases, and particularly oxygen and water vapour, through the materials. The required permeability levels to these gases are extremely low, and consequently the permeation study requires permeation measurement devices with high sensitivities.
Many permeation measurement devices have been developed in response to this problem, based on various gas monitoring principles, each with their disadvantages.
In particular, a device has been developed for measuring-permeation flows of O2, Ar or He, comprising a measurement enclosure in ultra vacuum. However, the sensitivity of this measurement method is limited by the residual pollution level of the monitored gas in the vacuum measurement enclosure, oxygen and water vapour present in the atmosphere naturally being among the greatest pollutants, even in a vacuum enclosure.
To overcome this severe disadvantage, it has been proposed to use an isotope of the target gas for which it is required to determine permeation through the material, using in other words a gas corresponding to the target gas but with a different mass number. Thus, by using a mass spectrometer as an analyser in the measurement enclosure for example, the permeation detection thresholds can be reduced by several orders of magnitude. Since the natural isotopic abundance of these elements is very low, pollution of the enclosure by these species becomes less important.
This measurement method increases the sensitivity of permeation measurements. Nevertheless, measurement times for target gases such as water vapour or oxygen are not at all lower than with conventional methods. These methods can measure the permeability of only one target gas through a film. Since the films and particularly films with a thin coating) are very fragile, any manipulation could cause an alteration to the film barrier properties (for example due to a scratch). If a change of the target gas requires modification of the instrumental device, it will often be necessary to replace the film for each measurement, which creates a lack of reproducibility and reliability of the measurements.
It is therefore a goal of the invention to propose improved device and method for measuring permeation, which in particular combine sensitivity and shorter measurement times.